The ISC issued the statement along with the publication of a large-scale report on the murder.
According to the report, Michael Adebowale, who, together with Michael Adebolajo, hacked Rigby to death on a London street, had an online exchange with a foreign extremist in December 2012, in which he expressed his intent to murder a soldier "in the most graphic and emotive manner." The ISC emphasized that the man would have become MI5's top priority had the data been shared with the Security Service.
"What is clear is that the one party which could have made a difference was the company on whose system the exchange took place … Therefore, even if MI5 had sought information — under a warrant — before the attack, the company might not have responded," the ISC said in the statement.
"They [companies] appear to accept no responsibility for the services they provide. This is of very serious concern: the capability of the [security] Agencies to access the communications of their targets is essential to their ability to detect and prevent terrorist threats in the UK," the committee stressed.
The company, which remains unnamed, does not consider itself liable for identifying or reporting such threats to respective government bodies, the ISC said.
"We find this unacceptable: however unintentionally, they are providing a safe haven for terrorists," the press release read.
"We are pushing them [companies] for that," Cameron stated, also pledging more financial support for UK intelligence to fight "self-starting" terrorism at home.
On Wednesday, the UK parliament will consider a new counter-terrorism and security bill granting new powers to the police and intelligence agencies. When announcing the bill, UK Home Secretary Theresa May described the current level of terrorism threat for Britain as "greater than it ever has been."
Lee Rigby, a UK army soldier, was killed in Woolwich, southeastern London on May 22, 2013. His murderers Michael Adebowale and Michael Adebolajo were sentenced to a minimum of 45 years and life imprisonment respectively.